Remote control or remote access systems use a transmitter and a receiver. An exemplary example is a garage door opener system where a transmitter is contained in a remote control unit and a receiver is connected to a garage door motor. The transmitter and receiver are typically comprised of different integrated circuits. When activated, the transmitter sends a data stream to the receiver. The data stream usually consist of two parts; a synchronization sequence (sync pulse) and a data sequence such as is shown in prior art FIG. 10. The communications channel is often of low quality and noisy so noise often precedes the synchronization sequence. Typically a transmitter has an identification code that is encoded into the serial digital data format when the transmitter is activated. The received data stream is typically fed into an adaptive filter. An adaptive filter can adjust its gain and offset and "synchronize" to determine the duration of one bit of information. Thus, the adaptive filter searches the format and sets a threshold to extract the data.
In prior art FIG. 10, typically the sync pulse is a single pulse that is very long when compared to the data pulse. This aids in differentiating between sync and data. As noise precedes the sync pulse, the threshold switching value of the adaptive filter in the receiver is adjusted to the average value of the noise level. This is too low for good detection. Eventually, the sync pulse comes along and the switching threshold switching level of the adaptive filter adjusts upwards. However, the adjusted switching level is often very high due to the length of the sync pulse. Ideally, a switching level at about the midpoint of the sync pulse is desirable. Additionally, the time constant or rise time of the adaptive filter is important. A faster rise time is desirable so that by the time that data comes along, the filter is able to switch effectively. The rise time of the filter with the long sync pulse is often too slow for good data detection.